Using both clinical research and population-based cancer registry in long-term research- a case study using EORTC trials and The Dutch National Cancer Registry (IKNL)

•Increased survival pleads for long-term follow up of key outcomes among patients treated for cancer.•Data collection beyond end of trial is challenging due to operational and ethical constrains.•Lacking of accurate treatment registration, registry-based studies often fail to draw causal relation between treatment and outcome.•This single disease, single registry case study showed clearly the complementary features of clinical and registry databases.•Collaborative proposals between clinical research and registries are put forward in long-term outcome research.